The ability of an electrolytic tin coating on the inside of an air-free steel shell to prevent corrosion due to the influence of food products contained therein has been known for many years, since 1937: See "Corrosion Resistance of Electrolytic Tin Plate," Part 2 by Kamm, Willey, Beese and Krickl, Corrosion, February, 1961.
In a series of later studies (1963 to 1965) the advantageous influence of tin deposited electrolytically was identified with a layer of continuous close-packed tin-iron alloy crystals lodged between the steel substrate and the free tin surface. The alloy was determined to be the result of melting ("flow brightening") the tin coating, conventionally accomplished by heating the tin matte strip rapidly from room temperature to above the melting point of tin in four seconds or less followed by a water quench.
From these studies emerged the concept of "K-plate" specifications related to factors of pickel lag, ISV and ATC. Pickel lag is a measure of the rate of pickling the low carbon steel metal preparatory to plating; ISV (iron solution value) is a corrosion test to measure the quantity of iron exposed through an electrolytically deposited tin coating on a low carbon steel substrate; and ATC (alloy tin couple) is a corrosion test to measure the rate of tin dissolution from the surface of an electrolytically deposited tin coating on a low carbon steel substrate exposed to mildly acid food products typified by grapefruit juice.
Low values of ISV and ATC characterize high resistance to corrosion, necessary for K plate quality employed for those food products having mild to high acid values likely to produce catostrophic corrosion. Historically, the factors of K-plate quality have been of immense value, particularly the ISV and ATC factors but for practical purposes of this invention the historic factors are supplanted by detinning measurements as a measure of K-plate quality.
The paper entitled "K Plate for Heavily Coated Electrolytic Tin Plate Applications" (Kamm and Krickl, Mechanical Working and Steel Processing, IV, Proceedings of the 8th Mechanical Working and Steel Processing Conference and Symposium on New Metal Forming Processes, Metallurgical Society, AIME, 63-101, 1965; published by Gordon and Breach, N.Y., 1969) recognizes extremely low ATCs are obtained by baking the tin plate at temperatures below the melting point of tin and that such treatment would be useful in advancing inferior grades of tin plate "to the K-plate level, were it not for the fact the alloy is not mended at pores in the tin coating." From this it was concluded that baking after melting was not the way for encouraging K-plate quality. Indeed, the article then proceeded to emphasize that the factors for encouraging K-plate quality, where it has not been achieved, would be in the realm of (a) preparation of the steel more readily receptive of tin plating, (b) pretreatments of the steel to remove oxide or contaminants from the steel surface, (c) pretreatments to promote alloy nucleation or, (d) melting techniques to overcome resistance to alloy nucleation. One aspect of nucleation will be discussed below. These conclusions where founded in part on data set forth in an earlier article entitled "Methods for Improving Corrosion Resistance of Electrolytic Tin Plate" (Kamm, Willey and Beese, Materials Protection, December, 1964.)
The 1964 and 1965 studies analyze in detail the solid phase alloy nucleation technique (SPAN Melting disclosed in Kamm U.S. Pat. No. 3,087,871) developed "from porosity studies which indicated that a matte tin deposit is more continuous than the same coating after melting." According to this technique, the close-packed continuous tin-iron alloy, deemed necessary for corrosion resistance, is sought to be achieved by heating the matte tin steel strip to a temperature of 420.degree.-450.degree. F. (below the melting point of tin) in a very short time, held there for about four seconds, then heating to above the melting point of tin within a second, followed by the quench.
Regardless, the authors of the earlier article (1964) found variable results depending on the quality of the steel. This finding prevailed even though severe increases in both time and temperature were adopted and in spite of purposely inducing melting both before and after treatment according to the Kamm patent. More importantly, it was found that trying to control the temperature within the very narrow limits required, coupled with the high cost of induction heating equipment to obtain those narrow limits, where insurmountable production problems. The conclusion was that superior tinplate from the standpoint of corrosion resistance would be achieved "by improving steel processing prior to the tinning line." This was confirmed by the authors of the later study, Kamm and Krickl, 1965.
(Conventional melting of a matte tin surface on steel is shown by FIG. 1 hereof and SPAN melting is shown by FIG. 2, identified hereinafter.)
It should be understood that the studies referred to above, and the Kamm patent as well, are concerned with achieving K-plate of the highest quality in order to safeguard the steel container against the corrosive influence of certain food products over a long period of time. Containers for packaging other food products do not necessarily require K-plate quality.
The efforts discussed above to achieve K-plate of exceptional integrity have been concerned with preparing matte tin steel plate for soldered forms of can containers characterized by three pieces, namely, a cylindrical body with a soldered side seam, a bottom and a top.
In the meantime, almost concurrently with those studies, commercial development of seamless steel containers materialized. A seamless container may be formed in two ways, principally. It may be achieved by draw-redraw or by draw-and-iron. In redrawing, a starting blank is first drawn to cup shape and then redrawn in successive steps progressively to lengthen the cylindrical side wall of the cup to develop a shell of the desired length. The metal for developing the side wall is obtained from the periphery of the blank and there is virtually no change in wall thickness compared to the starting blank. In draw-and-iron the starting blank is also initially drawn into a cup shape but progressive length is developed afterwards by a series of ironing dies (progressively of smaller diameter) which progressively thin the side wall to obtain metal for the lengthened shape, causing the side wall to become thinner and thinner in the course of developing a shell of the desired length. In both forms of production, the blank, flat to start with, is drawn to cup form and the circular side wall is progressively lengthened until a closed bottom shell of the desired length is obtained. A suitable shell of the seamless type may also be obtained by a mere draw in a single pass.
Until the derivation of the present invention, as best known, there has been no seamless, tubular, closed bottom food container which exhibits sufficient K-plate quality to preserve contents of high acidity (e.g. tomato juice and grapefruit juice) for prolonged periods. The reason, simply stated, is that during drawing the tin-iron alloy is drawn (worked) and ruptured. Continuity of the alloy is demolished and iron is exposed to the acidity of the packaged food once the sacrificial tin layer has been exhausted in the critical areas.